Among those eyeing a run: Ellen Meehan, the ex-wife of former Democratic Rep. Marty Meehan — who represented the district before Tsongas — and a former campaign chair for Tsongas.

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“This will certainly be a highly contested race as it has been in the past,” Meehan told POLITICO. “I’m going to give it very serious consideration.”

Tsongas called Meehan this morning to inform her of her decision not to seek reelection and held another call with donors this afternoon to inform them, which Meehan also participated in.

Within hours of Tsongas’ surprise announcement, the rare opening in the Massachusetts congressional delegation attracted several other prominent Democrats. State Sen. Barbara L’Italien and former state Sen. Barry Finegold, who finished fourth behind Tsongas in the 2007 Democratic special primary election, both said in statements that they are also considering running for the seat.

Two additional Democratic names from the 2007 special primary election are also surfacing. State Sen. Jamie Eldridge, who came in third place, told POLITICO he is “taking a look” at running. Another potential candidate is state Sen. Eileen Donoghue, who finished a close second to Tsongas.

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Another possible candidate in what’s expected to be an expanding field is Dan Koh, chief of staff to Boston Mayor Marty Walsh, who has made waves in Boston while maintaining a presence in his hometown of Andover.

While Tsongas typically won reelection by comfortable margins — and Hillary Clinton carried the 3rd District by more than 20 percentage points in 2016 -- Republicans insist the right GOP candidate has a shot at winning the open seat. The state party is weighing its chances in the district, which only gave Sen. Elizabeth Warren a narrow win in 2012 and was carried by Gov. Charlie Baker in 2014 and Republican Senate candidate Gabriel Gomez in 2013.

MassINC Polling Group President Steve Koczela described the Massachusetts 3rd — which includes blue-collar Fitchburg, along with reliably Democratic working-class mill towns of Lawrence and Lowell — as one of the more competitive of the state’s congressional districts for Republicans.

“All Massachusetts congressional districts are more blue than red, and there have been relatively few Republicans in the recent decades in Mass congressional delegations,” he said. “But as districts go, it’s one of the more potentially competitive.”

Among the Republicans looking at the seat is Rick Green, founder of the influential, conservative-leaning nonprofit Massachusetts Fiscal Alliance and owner of a Pepperell-based business.

“I’m sure he’s going to be putting some thought into it,” said Paul Craney, executive director of MFA. “[He] is clearly the frontrunner.”

Green briefly considered challenging Warren in next year’s Senate election.

The open seat contest could be buffeted by political crosswinds next year. Aside from the Senate and gubernatorial races at the top of the ticket, a potential 2018 ballot question that would reduce the state sales tax could prove especially popular for residents living in a district that stretches along the border of sales tax-free New Hampshire. And strong early organization around another ballot question that would increase taxes on annual income earners over $1 million could draw strong turnout among progressives.

“Any competitive district is an uphill climb [for Republicans in Massachusetts] and this cycle seems like nationally it’s going to be a good environment for Dems,” Koczela said. “That makes it more difficult for Republican pickup, but it appears to be the best opportunity Republicans have in the state.”